Add a quicker build instruction in README (openhab#4609)

Signed-off-by: Jimmy Tanagra <[email protected]>

Author: jimtng

README.md

@@ -42,10 +42,12 @@ To build this project from the sources, Maven takes care of everything:
 
 - set `MAVEN_OPTS` to `-Xms512m -Xmx1024m`
 - change into the openhab-core directory (`cd openhab-core`)
-- run `mvn clean install` to compile and package all sources
+- run `mvn clean spotless:apply install` to compile and package all sources
 
 If there are tests that are failing occasionally on your local build, run `mvn -DskipTests=true clean install` instead to skip them.
 
+For an even quicker build, run `mvn clean install -T1C -DskipChecks -DskipTests -Dspotless.check.skip=true`.
+
 ## How to contribute
 
 If you want to become a contributor to the project, please read about [contributing](https://www.openhab.org/docs/developer/contributing.html) and check our [guidelines](https://www.openhab.org/docs/developer/guidelines.html) first.